The present invention relates to planar slotted-waveguide antennas, of the type employed in radar.
For surface-based radars, especially those operating in the frequency range of 1-20 GHz, both reflector antennas and planar arrays with electronic scanning can be used for a variety of radar applications.
Reflector antennas have advantages of low cost and light weight but are basically limited in application to either a 2D surveillance mode (range and angle in the axis of rotation) or as dedicated single target trackers. Fully electronically scanned array antennas can provide the beam agility required for multimode operation throughout a quadrant of hemispherical space, but are relatively heavy and extremely expensive.
Midway between the above two approaches in the cost/capability range is a planar array with a stack of beams or with electronic scanning in one dimension. This often takes the form of a planar array of parallel slotted waveguide elements. Mechanical rotation provides scanning in either the azimuth or elevation plane with phase shifters and/or frequency changes to steer the beam electronically in the other plane, i.e., elevation or azimuth. The usual antenna implementation provides a pencil beam in the plane of the waveguide elements rather than monopulse. Although this type of antenna offers advantages of light weight, low cost and good maintainability, relative to a fully phased array, target location in the plane of the waveguide elements requires beam splitting by using the sequence of radar returns as the antenna beam rotates through the target position. This technique is less accurate than monopulse and more susceptible to electronic counter-measures (ECM). Furthermore, the need for multiple hits on each target to determine its location, as compared to monopulse which may require only a single return, curtails its "track-while-scan" applicability in a dense target environment. It is also not suited to dedicated tracking antennas that may combine electronic beam steering in one plane with mechanical slewing in the other to track multiple targets within some region of space.
Prior techniques for providing monopulse with edge-slotted waveguide antennas have usually involved a pair of pencil beams overlapped in angle and combined in a hybrid tee to obtain sum and difference outputs, e.g. Branigan et al. U.S. Pat. No. 4,958,166 and Wong. U.S. Pat. No. 3,430,247. However, practical problems of packaging the closely spaced sticks and obtaining acceptable monopulse patterns appear to have largely curtailed their application.
The monopulse principle involves radiating a pencil beam to illuminate a distant target, and then separately receiving and processing a sum return signal and a difference return signal. A pair of overlapping pencil beams can be generated in interleaved arrays or in slotted waveguides that operate in either of two different modes of excitation, with separate hybrid ports being provided for each mode. This is described in U.S. Pat. No. 4,164,742. However, the overlapping pencil beams there described achieve rather poor monopulse performance, because of non-ideal excitation of the arrays. Also, any departure from the design RF frequency can cause variation in the angular overlap of the beams for the interleaved arrays, as changing the beam squint (off-broadside look angle). This can further degrade the sum pattern beam width and the difference pattern error slope.
Ideal monopulse sum and difference amplitude tapers can be expressed in terms of even and odd aperture field components, respectively about the center of the aperture. As a practical matter this may be achieved by interleaving two arrays of slotted waveguide elements, one array providing the sum excitation and the other array providing the difference excitation. In theory, either array could provide the even (sum) excitation with the other array providing the odd (difference) excitation. The field components that contribute to the monopulse sum and difference signals are thus separated in the array aperture itself rather than in monopulse beamforming networks located behind the array elements. This is described, e.g., in Laverick et al. U.S. Pat. No. 3,636,563.
In contrast to antenna systems that form the monopulse excitations from a pair of overlapped pencil beams, the beam squint or look angle for both the sum-and-difference interleaved arrays can be designed to be identical. This has the advantage that waveguide propagation velocity and slot spacing for both arrays is the same; as a consequence, the sum and difference patterns remain coincident with changes in RF frequency. This gives the antenna an inherent performance advantage.
Despite these advantages, the interleaved sum-and-difference approach to slotted array antenna does have shortcomings. For one thing, only one array of elements, i.e., the sum elements, can be used on transmit for target illumination. With phase shifters required at the input to each slotted waveguide element for beam steering in the orthogonal plane (stick-to-stick), the maximum array transmit power is thus limited to the combined power handling capability of only one-half the phase shifters.